1. Field of the Invention
The invention relates to a silencer designed and intended for a compressor or a vacuum pump, in particular for a compressor that operates according to the displacement principle, or for a vacuum pump that operates according to the displacement principle, which in either case compresses a current of gas, in particular an air current, such that the silencer includes an entrance for the gas current that leaves the compressor, as well as an exit. Within the silencer there is provided a branching region at which an inflow channel divides into two channel sections, namely a first channel section constructed as a main conduit to conduct the gas current further, and a second channel section forming a branch that is closed at its end. The invention further relates to a compressor equipped with such a silencer, as well as to a method of reducing pulsations in a gas current generated by a compressor.
2. Description of the Related Art
A silencer with the characteristics described above is already known from the patent EP 0 542 169 B1. In general, compressors, in particular those that operate according to the displacement principle (e.g., screw-type compressors, Roots blowers), present the problem that because the expulsion process on the pressurized or expulsion side of the compressor is discontinuous, pulsations arise in the downstream components, such as pipelines, coolers, containers etc., and these in turn give rise to two main problems, descriptions of which follow.
First, the attached components are placed under considerable stress by the change in pressure, which can cause damage to their material owing to fatigue (permanent fractures, etc.) caused by the cyclic loading produced by the primary pressure change and/or the associated oscillations.
Second, the considerable noise emission resulting from the pressure change also as a consequence of the initiation, conduction and radiation of structure-borne sound, proves to be extremely disadvantageous. These problems are especially severe in the case of screw compressors that operate under dry conditions, so that pulsations that may be quite large are produced at the exit from the compression stages. Because the expulsion processes are not harmonic in nature, i.e., sinusoidal or cosinusoidal, but rather are impulse-like, the harmonics of the basic frequency are also emphasized in the frequency analysis, in some cases even more strongly than the basic frequency.
The pulsations with an amplitude relevant here are as a rule within a broad frequency range, typically 200 Hz to 10 kHz. Because of the tonal character of the pulsation (main expulsion frequency and its harmonics) the noises thus produced are subjectively unpleasant.
The main expulsion frequencies can vary widely within a compressor series composed substantially of identical components, on account of various influences. For one thing, the output is often adjusted by regulating the speed of rotation, for instance by means of a frequency converter. Furthermore, individual compressors are often delivered with differently constructed gear trains to drive the compressor stages, in order to adjust the power/pressure. Finally, within a compressor series some individual compressors operate while connected to 50-Hz electrical networks, and others are connected to 60-Hz networks.
Sound-damping procedures effective within a narrow band, with slight dissipation, are not well suited to operate under the limiting conditions described above, because either a plurality of differently tuned silencers are needed for a single compressor, in order to achieve a certain broad-band action, or several silencer variants must be supplied, which are then matched to the individual compressor variants or their later application circumstances. However, this is possible only if the compressor involved is not one having variable frequency (e.g., rotation-speed regulation).